This invention is concerned in general with an automatic gain control (AGC) circuit for a radio receiver and, more particularly, with an extended AGC circuit which biases one of a pair of back-to-back diodes, normally used to clip high RF input signals to the radio receiver, into a continuously conductive mode to extend the AGC performance of the radio receiver under high level RF input signals, and further with biasing of the back-to-back diodes to obtain local/distant switch operation in the radio receiver.
Use of back-to-back diodes to clip the RF input signal to a radio receiver is known to the prior art. Generally, such back-to-back diodes are disposed at some point in the radio receiver between the antenna input and the first RF amplifier stage. Typical silicon back-to-back diodes operate to clip the amplitude of the input RF signal to about .+-.0.7 volts. Basically, the diodes limit the signal by being rendered conductive during that portion of the RF signal which exceeds .+-.0.7 volts by shunting the peaks of the RF signal to a DC reference, usually ground potential. Under such operating conditions, the back-to-back diodes are not conductive during those portions of the RF signal which do not exceed .+-.0.7 volts. Of course, if the peaks of the RF signal significantly exceed .+-.0.7 volts, the back-to-back diodes may not be able to limit the peak-to-peak magnitude of the RF signal to .+-.0.7 volts, but instead will limit the RF signal at some higher peak-to-peak magnitude. Back-to-back diodes are often used in transceivers as peak signal limiting devices to prevent any transmitted signal from entering the receiver portion of the transceiver with excessive magnitude.
It is also known to the prior art to use an automatic gain control (AGC) circuit in the radio receiver to control the gain of various portions of the radio receiver in response to the strength of the RF input signal. Generally, the AGC signal is generated in the intermediate frequency (IF) portion of the radio receiver as a DC level which is related in magnitude to the magnitude of the IF signal, which is in turn related to the magnitude of the RF signal. This DC level of the AGC signal is then usually applied to the RF amplifier portion of the radio receiver to control the gain of the RF amplifier. This enables the radio receiver to be responsive to a wide range of RF input signals without generating a correspondingly wide range of detected and recovered audio signals. That is, due to AGC, the magnitude of the audio signals will generally be relatively constant, or at least not vary over correspondingly broad ranges, in comparison with the broad range of RF signals which may be applied through the antenna to the radio receiver. A widely known shortcoming of such AGC circuits is that the AGC circuits are not responsive to a sufficiently broad range of input signal levels to accommodate all of the possible RF input signal magnitudes, especially at high RF input signal conditions.
It has also been known to the prior art to use local/distant switches to give the operator of the radio receiver more control over the performance of the radio receiver under high RF input signal conditions. This is because the local/distant switch, when in the local mode, typically inserts some attenuation between the antenna and the input to the RF amplifier stage of the radio receiver such that high RF input signals are significantly attenuated. The radio receiver is then capable of handling the attenuated RF signals. In the distant mode of the local/distant switch, little if any attenuation is inserted into the antenna lead to the RF amplifier stage and the radio receiver therefore operates in its usual manner of handling the RF input signals directly from the antenna. Performance of the radio receiver in receiving weak distant signals is thus not degraded.
The back-to-back diode circuit, the AGC circuit and the local/distant switch function of the radio receiver have heretofore typically been separate circuits which have not been functionally interrelated or interactive.